The invention concerns generally a process for the production of a connecting element of thermoplastic material for connection to a hollow body also of thermoplastic material, and more particularly, but not exclusively, production of a component in the form of an insert which is adapted for lining a connection opening of a fuel lank and/or to form a seat for a connection element.
A fuel tank, for example for a motor vehicle, may typically include a component which lines a connection opening of the tank and/or which forms a seat for a connection element. Connection elements in that respect may be, for example, valves, nipples, connection sleeves and like members. Furthermore, a connection opening of the tank can be designed to accommodate a delivery or conveyor unit or it can be in the form of a sender opening for accommodating a sender for indicating the level of fuel in the tank, or in the form of a lead-in tube connection.
High demands are to be made on fuel tanks, in terms of their permeability in relation to hydrocarbons. In accordance with the most recent regulations relating to environmental protection, the total level of hydrocarbon emissions of a vehicle in the non-operating condition are to be not more than 0.5 g in a period of 24 hours. Correspondingly high demands are to be made in terms of the maximum level of hydrocarbon emission of a fuel tank system. For that reason fuel tanks may often be made by a co-extrusion blow molding process. wherein the wall thereof consists of a laminate with one or more barrier layers affording a low level of permeability to hydrocarbons. Such tanks generally satisfy the enhanced demands in regard to the maximum permissible levels of hydrocarbon emissions.
Problems are involved in this respect however in regard to the connection openings in the tank, which form a sealing seat for valves, connection portions and the like. As such connection openings have to involve close tolerances in order for example to guarantee a sufficiently sealing and play-free fit of a connection portion or valve therein, it is in many cases not possible to also provide the valve seat or sealing seat for the connection component in one working operation in production of the tank by extrusion blow molding.
Accordingly, an opening is provided in the tank, to act as the connection opening. There are now various possible ways of inserting, for example, a valve into that opening in sealing relationship therewith. On the one hand, there is the possibility of screwing the valve against the tank wall, with the interposition of seals, which also bear against the tank wall structure. That arrangement is unsuitable for fuel tanks by virtue of the enhanced demands in terms of sealing integrity of the tank. Welding the valve to the tank also cannot be readily considered as valves and connection portions generally consist of POM (polyoxymethylene), in particular because of the need for the material to be resistant to the fuel, whereas the tank at least partially comprises PE (polyethylene). The various materials involved are not weldable to each other.
For that reason, nipples or valves consisting of POM may be provided with a stem which is injection-molded from PE and into which the valve or the connection nipple is fitted by clipping engagement. The stem can then be welded to the tank in the region of the connection opening thereof, for example by means of a hot plate welding procedure.
That manner of manufacture, however, still suffers from the disadvantage that the connection opening of the tank represents a weak point in regard to the possible emission of hydrocarbons as PE (polyethylene) is permeable in relation to hydrocarbons and swells up in the presence thereof whereas POM (polyoxymethylene) is scarcely pervious to hydrocarbons. A connection opening in the tank therefore still constitutes a source of emissions.